Geological formations forming a reservoir for the accumulation of hydrocarbons or other fluids in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed that may ingress or egress from the reservoir. To determine the behavior of the fluids in this network, knowledge of both the porosity and permeability of the geological formations is desired. From this information, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the resistivity of geological formations is a function of both porosity and permeability. Considering that hydrocarbons are electrically insulating and most connate water contains salts and is highly conductive, resistivity measurements are a valuable tool in determining the presence of a hydrocarbon reservoir in the formations.
One technique to measure formation resistivity involves the use of electromagnetic induction using transmitters of low frequency magnetic fields which induce electrical currents in the formation. These currents in turn produce secondary magnetic fields which are measured by a magnetic field receiver.
The magnetic field receiver can be placed in a wellbore when performing a cross-well electromagnetic survey or surface-to-wellbore or wellbore-to-surface electromagnetic survey. A wellbore is typically lined with casing, which is usually made of steel. The magnetic permeability of steel casing is often non-linear and depends on a magnetizing field and frequency. The casing effect on a magnetic field receiver is strongly dependent on the magnetic permeability of steel casing. Since the magnetic property of steel casing can be modified by induced current from a source coupled through a formation during an electromagnetic induction survey, the effective sensitivity of the receiver inside the casing can also change. As a result, currents induced into casing may cause data distortion of electromagnetic induction surveying using receivers positioned in steel cased wellbores.